Murphy brake

The Murphy brake is a drum brake with shoes that pinch the drum via a rotating fork. In contrast, most drum brakes use either shoes that press only on the inside of the drum, or shoes that press only on the outside of the drum, like a band brake. Like many drum brakes, the Murphy brake is self-assisting, so a low application force gives a high brake force. Like a disk brake, and unlike most drum brakes, the Murphy brake uses short shoes so braking force is less sensitive to changes in the coefficient of friction, for example as the drum and pads heat up. By using a "pinch" configuration, a relatively light drum can tolerate high pad forces, thus allowing use of pads with low coefficient of friction. The drum is slightly tapered or "bell mouthed", and the shoes are mounted movably so the brake drum can change shape as it heats and the pads will follow the change, without interfering with the brake's operation. The brake is named after its inventor, John H. Murphy.

Design and operation

The Murphy brake ^{[1][2][3][4][5]} clamps a drum with two shoes. The shoes are held by a two-tined "fork" that rotates to pinch the shoes on the drum. As the fork rotates, the shoes also move in opposite directions along the surface of the drum, with the "leading" shoe moving in the same direction as the drum, and the "trailing" shoe moving in the opposite direction. When the shoes touch the drum, they are pulled by the drum in the direction of rotation. The fork does not resist this pull; instead, the trailing shoe is pulled against a stop, and the leading shoe is held by pulling on the fork, which in turn pulls on the trailing shoe and its stop. Because the leading shoe pulls through the fork, it also applies the brake harder, called "self assist". The amount of assist depends on the angle of the fork: if the fork tines are nearly radial, gain is high. If the fork tines are nearly tangent, gain is low.

As the brake heats, the drum tends to grow in diameter. Since it is a drum, it is constrained at one end, and with increasing temperature, the drum gets more and more conical. The fork is mounted so the shoes can move radially, and either the fork or the shoes are movable to follow the bell-mouth shape.

Self-assisting brakes tend to suffer from at least two related problems: grabbiness/chatter and fade. Grabbiness often occurs with a high-gain self-assisting brake and with light rust on the brake. Fade often occurs when the brake is used hard and gets hot. Both problems can be severe: in the days of all-drum car brakes with high boost, drivers would often find the first brake application of the day would cause wheel lockup, because of light rusting that occurred overnight. Conversely, drivers were often warned to gear down and use engine braking on all but short hills, lest the brakes got hot and became useless. Even in 2010, most U.S. and third-world heavy trucks use drum brakes, so drivers must use engine braking extensively—yet they still suffer occasional runaway brake failures due to heating.

Both problems are more common and more severe when the brake has long shoes. Many self-energizing brakes are described by the formula e^μθ, where e is the natural logarithm base, μ is the coefficient of friction between shoes and drum, and θ is the angle of wrap. If μθ is large, then the brake is strong, but small changes in the coefficient of friction (μ) lead to large changes in brake strength. Conversely, if μθ is small, the brake requires a harder application to reach the same level of braking, but the brake is less sensitive to changes in friction.

The Murphy brake uses short shoes, thus a small θ. Further, Murphy's tests often used a pad material with low friction but little change in friction when hot, such as sintered bronze. The small angle and low pad friction require higher application force or higher gain to achieve strong braking, but reduce the brake's tendency to grab and fade. In principle, a Murphy brake can be built with enough gain that it can be operated directly, without expensive hydraulics or servo boosters.

Murphy brakes use a drum that is stamped or spun from flat steel plate. The drum is made slightly conical to reduce construction costs, and the use of stamping or spinning can reduce cost compared to cast iron or steel used for conventional drums or ventilated disk brakes. Lightweight disk brakes are made directly of flat steel so can be yet cheaper, but discs for heavy vehicles internally vented and thus more expensive.

Another goal of the Murphy brake is other brake parts can be held with low precision, thus reducing total brake cost.

The basic "tilting fork" mechanism could be used with a disc instead of a drum. Murphy used a drum because it can be heated to red hot and will grow in radius but does not distort irregularly; in contrast, a light disk is prone to develop S-shaped distortion when hot. Murphy also chose a drum because a drum inside a wheel has more cooling area than a disc in the same space. A disc might be preferable where lateral space is constrained but radial space unconstrained.

A few possible problems with the Murphy brake may have been solved, however Murphy is not readily found and may have since died, so details are unknown. One problem is the shoes are mounted relatively loosely, so in bounces may contact the drum, causing drag or spurious brake application. Some kind of sprung or elastic mount may be sufficient, but details are unclear.

Another possible problem is the fork angle changes as the pads wear. Worn pads require the fork to turn more, and thus self-assist gain goes down, causing the brake to get weaker as the pads wear. The higher the gain, the more sensitive it is to wear. Wear compensation may be possible, but needs to be within the fork/pad assembly.

Tests

Murphy described several tests he performed. Although there were reportedly third-party viewers, the only information presently available about the tests was via Murphy himself.^[6]

Logging truck test

Logging trucks on mountains routinely suffered brake failures due to overheat. Water cooling had been tried with good results but the steam tended to blind other drivers, and running out of water still led to brake failures.

Murphy fitted four drums, roughly 30 cm (12") diameter, to a logging truck. The brakes were operated via a mechanical linkage from the driver's pedal, rather than the more conventional air or hydraulic and had it driven down a mountain where brake failures were common, ultimately with a full load of logs plus the engine being used to drive the truck rather than brake it. At the bottom, the brake drums were glowing dull red-orange and the pads had blackened due to heat, but the brakes were still effective for stopping.

Tank test

Two drums, roughly 40 cm (16") were fitted to the drive wheels of an Army tank. The brakes were operated by a hydraulic line connected to the driver's controls.

The tank was driven through or parked in various substances including mud and cold water, then parked long enough for ice to freeze. The brakes were then tested, and were found to operate reliably, smoothly, and with enough power to lift the rear portion of the tracks off the ground.

Bus test

A Murphy brake was fitted to the transmission of a front-engine school bus and operated via a mechanical linkage. A bus has an unusually long driveshaft, which is somewhat springy, so tends to exaggerate grabbiness: a grabby brake makes the driveshaft wind up, rather than turning the brake. When the driveshaft is sufficiently wound up, it overcomes the brake, which suddenly "lets go", leading to pulsing brakes or skids. The Murphy brake used sintered bronze shoes for smoothness.

The bus was tested for panic stop braking distance and was tested with both cold and hot brakes. With the Murphy brake on the transmission, the stopping distance under hard braking was the same as with wheel-mounted drum brakes.

Commercial use

The Murphy brake was used as a transmission-mounted parking/emergency brake on an American-made light truck in the early 1970s.^{[citation needed]}

A small delivery van company used Murphy brakes in trials and may have used them in production; the name of the company is presently lost.

References

1. ^ U.S. Patent 2,700,437
2. ^ U.S. Patent 2,710,675
3. ^ U.S. Patent 2,783,858
4. ^ U.S. Patent 2,787,340
5. ^ U.S. Patent 4,039,052
6. ^ Milton W. Raymond, 1982.

External links